Cell reduction of bauxite or clay



United States Patent 3,507,643 CELL REDUCTION OF BAUXITE 0R CLAY Curtis J. McMinn and Vaughn L. Bullough, Florence, Ala., and Tom Winfield Williams, Little Rock, Ark., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed Jan. 16, 1967, Ser. No. 609,336

Int. Cl. C22c 21/00; C22d 3/12 US. Cl. 7563 13 Claims ABSTRACT OF THE DISCLOSURE Aluminum-titanium alloys are produced by direct reduction of titanium-containing aluminous materials such as bauxites or clays in a fused salt electrolyte, thereby forming a reduction alloy, then maintaining the reduction alloy in molten state at a temperature of about 700-750 C. to eilect separation into a supernatant phase containing about 0.3% Ti, and a solid phase containing upwards of 10% Ti, centrifugally separating the phases, and forming a ferrotitanium by conversion of the solid phase with iron oxide.

This invention relates to a novel method for the production of alloys of aluminum and titanium by the direct reduction of aluminous ores containing titanium in an electrolytic reduction cell. Moreover, the invention concerns the control of the titanium content of the aluminumtitanium alloys obtained by direct ore reduction.

In the conventional industrial process for the production of aluminum metal, aluminous ores, such as bauxite, are first converted into alumina, the feed material for the reduction cells. The basic Bayer process is employed for the preparation of. the intermediate alumina, comprising digesting the bauxitewith caustic soda solution to form a supersaturated solution of sodium aluminate from which relatively pure aluminum hydroxide is precipitated. The latter is then converted to alumina by heating in rotary kilns at about 1800 F. to drive off combined water. The insoluble constituents of the ore remain as a waste residue.

Aluminum metal is conventionally produced from alumina in electrolytic reduction cells by passing a current through a bath of molten electrolyte containing dissolved alumina in a large tank lined with carbon which serves as the cathode. Large carbon blocks presented at the top of the bath function as anodes. Molten aluminum metal at a temperature of about 1800 F. collects at the bottom of the cell and is removed periodically.

It is well recognized in the art that the Bayer process adds very materially to the cost of production of the final aluminum and methods have long been sought whereby bauxite or clay could be added directly to the reduction cells, eliminating the intermediate production and purification of alumina.

The production of pure aluminum directly by reduction of bauxite or aluminous clays in carbon lined electrolytic cells is difficult and uneconomical due to low current efficiencies which ordinarily prevail.

It has been found, however, in accordance with the present invention, that the characteristics exhibited by these cells in the direct reduction of aluminous ores can be turned to advantage, by employing such reduction for the production of aluminum-titanium alloys.

In accordance with the method'of the invention, high titanium aluminous ores, such as bauxites and clays, are subjected to reduction by electrolysis in a fused salt electrolyte, to produce a reduction alloy of aluminum, containing small amounts of titanium, silicon, and iron. The cell feed can also be a synthetic mixture of alumina or an aluminous ore and a titanium oxide such as rutile,

3,507,643 Patented Apr. 21, 1970 so as to adjust the proportion of titanium in the initially produced reduction alloy.

The proportions of silicon and iron in the reduction alloy are substantially the same as in the bauxite feed. The titanium content is generally equivalent to that of the feed, unless sufficient titanium is added to exceed the solubility of titanium aluminide formed in aluminum at the reduction cell operating temperature.

As explained more fully below, the titanium reduction alloy can be utilized directly as a starting material for the preparation of ferrotitanium by thermic reduction with iron oxide. The reduction alloys can also be employed for the production of rolled aluminum alloy sheet from suitable cast particles, as described in US. Patent 3,076,706.

Depending upon the type of starting material employed, the reduction alloy will generally have a silicon content of about 5 to 7.5%, iron content from 1 to 2.5 and titanium content from 0.3 to 2.0%, balance essentially aluminum.

In accordance with another aspect of the present invention, it has been found that the titanium content of the reduction alloy can be adjusted to any desired value down to about 0.3% by means of a controlled cooling step. The solubility of titanium in the aluminum reduction alloy is about 2% at 1000 C. and about 0.3% at 700 C. By employing this difference in solubility, the reduction alloy is treated, in accordance with the invention, by maintaining it in molten condition in a holding furnace, whereby separation into two phases takes place. By allowing the molten reduction alloy to cool to about 700-750 C. a supernatant phase is obtained which contains about 0.3% titanium. There is also obtained a crystalline intermetallic aluminum-titanium compound,

which may contain from 10% to 30% of titanium or higher. The supernatant reduction alloy can be separated from the intermetallic compound by centrifuging or other suitable means.

If the cell feed also contains enough silica to produce a hypereutectic aluminumsilicon alloy (silicon content in excess of about 12%), the titanium will also settle as a high melting intermetallic compound of titanium and silicon, or as a mixture of the two high melting intermetallic compounds. It is desirable to keep the silicon content of the alloy below about 12%, preferably 7% or less.

The practice of the invention will be illustrated with respect to the utilization of titanium bearing British Guiana bauxites, but it is to be understood that this is not to be regarded as limiting.

A typical titanium bearing bauxite would have the analysis:

Percent A1 0 91.1 SiO 4.80 Fe O 1.54 TiO 2.48

Upon electrolytic reduction of this bauxite in a conventional aluminum reduction cell, the reduction alloy produced would have the composition:

Percent The silicon and iron content of the reduction alloy result from the silicate and iron oxide equivalent which is present in the particular grade of bauxite which is reduced. However, the titanium content of the reduction alloy is controllable in accordance with the method of the invention.

Alloys with varying titanium content may be prepared by the choice of holding furnace temperatures used in subsequent treatment of the initial reduction alloy. The normal titanium concentration in the metal as tapped from the reduction cell is from about 1.6% to 1.8%, and this titanium content may be retained if an adequate temperature is maintained in handling the alloy. If lower titanium concentrations are desired, successively lower temperatures are employed in the holding furnace treatment to decrease the titanium content of the alloy.

It was found further, in accordance with the invention, that in the operation of a reduction cell employing titanium bearing bauxite as a cell feed, in addition to the reduction alloy which collects at the bottom of the cell and can be tapped therefrom, a ledge composed principally of titanium aluminide and aluminum accumulates around the cell cavity. Although this ledge is believed to contribute to reduce current efiiciency, it can be periodically removed and recovered, and subjected to the titanium adjustment treatment of the invention. Alternatively, the recovered low-titanium supernatant phase can be recycled to the cell for further removal of titanium in order to reduce this ledging effect.

The practice of the invention is illustrated by the following examples, which are not, however, to be regarded as limiting.

EXAMPLE 1 Manufacture of reduction alloy Percent Al 91.4 Si 4.

Fe 2.3 Ti 1.6

The voltage drop across the cell was approximately 3 to 5% above that normally used for alumina reduction. The reduction alloy was tapped from the cell into an ingot mold using a vacuum siphon.

EXAMPLE 2 Adjustment of titanium content The ingot obtained as described in Example 1 was remelted in a gas fired holding furance and brought to a temperature greater than about 950 C. to promote bath and dross separation from the metal, but less than 1000 C. to minimize oxidation of titanium and other alloy components. The bath and dross accumulation was skimmed from the surface of the melt. The melt was then stirred to insure homogeneity of the components. At this temperature, the titanium content of the reduction alloy was retained at about the original concentration.

(a) A portion of the melt was poured into the bowl of a hot metal centrifuge that had been preheated to about 800 C. The metal in the centrifuge bowl was allowed to stand as a quiescent pool until it had cooled to a temperature of about 750 C. At this point the sides of the centrifuge bowl were removed and the fine network of crystalline platelets of titanium-aluminum intermetallic compound was drained relatively free of supernatant metal by slowly increasing the centrifugal force on the crystalline network that had formed in the centrifuge bowl. The intermetallic compound that remained was a friable mass which was easily reduced to a particle convenient for handling in conventional mechanical grinding and crushing equipment. This cake had an average analysis of about 23% titanium, 6% silicon, and 70% aluminum. The supernatant metal drained from the centrfuge cake contained only about 0.4% titanium.

(b) A portion of the melt was allowed to cool to about 750 C., skimmed, and the supernatant metal containing about 0.3% titanium and about 7% silicon was decanted from the body of crystalline intermetallic titanium-aluminum compound formed. The intermetallic compound and entrapped supernatant metal produced an alloy containing about 16% titanium and 9% silicon.

(c) A portion of the metal was cast into particles by pouring it into a rotating cast iron cylindrical spinner 2% in diameter which had holes of .100 in diameter drilled in the sides of the spinner. With the spinner rotating at 1260 r.p.m., the molten metal was discharged through the spinner holes by centrifugal force into a layer of foam produced by agitating a water layer containing from 10-20 ppm. of sodium alkyl sulfonate detergent.

The resulting particles were 8 +20 mesh (US. Standard), balance +8 mesh, and were approximately 60% acicular in shape. The remaining 40% varied in shape from acicular to spherical. The particles were then dried, preheated to a temperature greater than 450 F., but less than 1200" F., and fed to a rolling mill wherein they were compacted into solid metal sheets .030" to .050" thick.

EXAMPLE 3 Production of reduction alloy A standard alumina reduction cell was operated an British Guiana bauxite feed for 25 days. The total amount of bauxite added was 40,600 pounds, producing 19,810 pounds of reduction alloy.

EXAMPLE 4 .Recovery of ledge alloy After prolonged service, the reduction cell of Example 1 was removed from service. The liquid contents of the cell were removed by siphoning, exposing a ledge of high melting titanium-aluminum intermetallic compound that had formed beneath the metal pad of the cell during the prolonged operation. This crystalline layer was removed from the cell with pneumatic hammers and the cell was returned to service. Analysis of the ledge material showed a titanium content of about 23% and a silicon content of about 5%.

EXAMPLE 5 Conversion of ferrotitanium The high melting intermetallic compound prepared by centrifuging as described in Example 2(a) was admixed with powdered iron oxide and calcium oxide in a magnesia crucible. This mixture was heated in an induction furnace to a temperature of approximately 900 C. at which point a strong exothermic reaction began. After cooling, a solid layer of ferrotitanium (6.4% titanium, 84.3% iron) was recovered. In order to increase the titanium content of the ferrotitanium, the admixture of intermetallic compound and metal oxides can be sup plemented by the inclusion of titanium dioxide. By this means, a ferrotitanium was produced analyzing 19% titanium and 71% iron.

While the presently preferred practices of the invention have been described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Method for the production of aluminum-titanium alloys by the direct electrolytic reduction of titaniumcontaining aluminous materials comprising the steps of:

(a) subjecting a titanium-containing aluminous material to reduction by electrolysis in a fused salt electrolyte to produce an aluminum-titanium reduction alloy;

(b) maintaining said reduction alloy in molten state at a temperature between about 700 and 750 C. for a period of time sufficient to cause separation into a supernatant phase comprising aluminum and titanium and a phase of higher titanium content comprising an intermetallic aluminum-titanium compound; and

(c) separating said supernatant phase from said intermetallic compound phase.

2. The method of claim 1 in which the reduction alloy produced contains from about 0.3% to about 2.0% titanium.

3. The method of claim 1 in which the supernatant phase contains about 0.3% titanium.

4. The method of claim 1 in which the intermetallic compound contains at least about titanium.

5. The method of claim 1 in which the aluminous material is a high titanium bauxite.

6. The method of claim 1 in which the aluminous material is a synthetic mixture of alumina and a titanium ore.

7. The method of claim 1 in which the supernatant phase is separated from the intermetallic compound by centrifuging.

8. The method of claim 1 in which the separated intermetallic compound is heated with iron oxide and calcium oxide to form ferrotitaniurn.

9. The method of claim 1 in which at least a portion of said supernatant phase is recycled to step (a).

10. The method of claim 1 in which the silicon content of said reduction alloy is maintained below about 12% by weight.

11. The method of claim 1 in which a ridge of high melting titanium-aluminum intermetallic compound is formed during the electrolysis step (a), and said ridge is periodically removed from the electrolytic bath.

12. The method of claim 11 in which said recovered ridge intermetallic compound is added to separation p 13. The method of claim 8 in which titanium dioxide is included in the reactants in order to increase the titanium content of the ferrotitanium.

References Cited UNITED STATES PATENTS 1,020,517 3/1912 Rossi -138 2,162,938 6/1939 Comstock et al. 7568 X 3,254,988 6/ 1966 Schmidt et a1 75-68 3,257,199 6/1966 Schmidt et al 75-68 3,374,089 3/1968 Robinson et a1. 7568 FOREIGN PATENTS 277,702 5/1928 Great Britain. 797,262 6/ 1958 Great Britain.

OTHER REFERENCES Metals Handbook, published by American Society for Metals, Cleveland, Ohio, 1948, p. 1167.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 

